The present invention relates to a transfer roller, more specifically to a transfer roller in which an elastic foamed material layer less scattered in a size of a cell and having a uniform cell diameter is provided on an outer circumference of a shaft and which can give a good transferred image.
In an electrophotographic apparatus and an electrostatic recording apparatus such as a copying machine and a laser printer, a transfer roller has so far been used as a member for electrifying a recording medium such as paper, OHP (overhead projector) and a belt to transfer a developer onto this recording medium. For example, in a color copying machine, in an image-forming method in which a charged insulating toner image carried on an image carrier is primarily transferred onto an intermediate transfer medium and in which this is then secondarily transferred onto a recording medium from the intermediate transfer medium, used is a transfer roller for secondary transfer for electrifying the recording medium to transfer a toner image on the intermediate transfer medium onto the recording medium.
In a laser printer, in an image-forming method in which a charged insulating toner image carried on an image carrier is transferred onto a recording medium, used is a transfer roller for electrifying the recording medium to transfer the toner onto the recording medium from an electrostatic latent image visualized by the toner.
An apparatus shown in the following FIG. 1 can be given as the example of the image-forming apparatus such as the laser printer. FIG. 1 is a schematic drawing showing one example of an image-forming apparatus such as a laser printer using a transfer roller, and a photosensitive drum 1 is uniformly electrified by means of a charger 2. Next, an electrostatic latent image is formed by means of an image exposing system 3 and visualized by means of a developing device 4 to become a toner image. When the toner image on the photosensitive drum reaches a contact area with a transfer roller 5, the toner image is pressed onto a record paper 10 which is transported synchronizing therewith, and voltage is applied on the transfer roller 5 from a bias electric source 8, so that the toner image is transferred onto the record paper 10 from the photosensitive durm 1. In the drawing, a code 6 is a bearing; 7 is a spring; and 9 is a cleaner.
The transfer roller used for this image-forming apparatus has a structure in which an elastic layer is formed on an outer circumference of a shaft usually comprising a good conductive material such as metal using a material obtained by providing a principal material including a rubber-like elastic material such as silicon rubber, NBR and EPDM and a polyurethane foam with conductivity by blending with a conductive agent. In this respect, a hardness of the elastic layer has to be low in order to obtain a uniform nip between a photosensitive drum or an intermediate transfer medium and a transfer roller and to obtain a suitable transfer amount which meets a high speed of the apparatus.
In such transfer roller, when an elastic foamed material layer is provided as the elastic layer described above, an average cell diameter thereof and a scattering range in the cell diameter are very important. If the average cell diameter deviates from a prescribed range, it is difficult to obtain a good transferred image having no image unevenness. If a scattering range in the cell diameter is large, the roller resistance becomes uniform, and scattering in light and shade is liable to be caused in a density of the transferred image. Further, if ones having a large cell diameter and a small cell diameter are present in a mixture, brought about is the unfavorable situation that the transferred image is striped in a certain case.
Under such circumstances, an object of the present invention is to provide a transfer roller having an elastic foamed material layer provided on an outer circumference of a shaft, which does not cause scattering in light and shade in the image density and does not make the transferred image striped and which provides a good transferred image having no image unevenness.
Intensive researches repeated by the present inventors in order to develop a transfer roller having the excellent performances described above have resulted in finding that a transfer roller provided with an elastic foamed material layer which has an average cell diameter falling in a specific range and in which scattering in the cell diameter falls in a specific range on an outer circumference of a shaft can meet the object thereof. The present invention has been completed based on such knowledge.
That is, the present invention provides a transfer roller comprising a shaft and an elastic foamed material layer provided on an outer circumference of the shaft and electrifying a recording medium to transfer a developer onto the above recording medium, wherein the above elastic foamed material layer has an average cell diameter of 300 to 400 xcexcm, and at least 80% of the whole cells has a cell diameter falling in a range of the average value xc2x1200 xcexcm. Further, the present invention provides an image-forming apparatus equipped with this transfer roller.